1. Field of the Invention
The present invention relates to a method for producing a thermal barrier layer or coating for a metallic component, and also to a corresponding barrier layer or coating for a metallic component.
2. Discussion of Background Information
Turbomachines, such as stationary gas turbines and aircraft engines, are operated at ever higher temperatures for the purpose of boosting efficiency, meaning that components in the hot gas-affected region of the combustion chambers and of the high-pressure turbine require protection by means of active cooling and of thermal barrier coating (TBC) systems.
Known TBC systems encompass a metallic adhesion and/or antioxidation coat based on MCrAlY, where M stands for a metal such as nickel, cobalt, or iron. As a result of the corresponding chromium and aluminum fraction, a coat of this kind offers protection from oxidation and improves the adhesive strength of a ceramic coat, which is disposed on the outside in the thermal barrier coating system and which customarily consists of yttrium-stabilized zirconium oxide (YSZ).
Although thermal barrier coatings of this kind already have outstanding properties, there is a problem in that aircraft engines can be subject to massive sand and dust exposures, such as ash particles or industrially induced dusts, for example. On the basis of their composition, such sand and dust exposures are also referred to as CMAS (calcium magnesium aluminum silicates).
At high temperatures, such dusts become liquid, and then interact with the thermal barrier coating by infiltrating micro-fissures or open pores in the outer ceramic coat of the thermal barrier coating. As a result there is embrittlement and corresponding failure of the thermal barrier coating and/or in particular, of the ceramic coat, meaning that the components protected by the thermal barrier coating must be replaced and/or repaired. This results in considerable effort and cost, and so the thermal barrier coatings require protection against CMAS damage.
Already known are measures for protecting the thermal barrier coatings from CMAS damage: through arrangement of reactive oxides above or in the outer region of the ceramic coat of the thermal barrier coating, the intention is to produce protection by using reaction of the liquefied CMAS with the reactive oxides to form a stable crystalline layer on the thermal barrier coating, this layer providing protection from further CMAS attack. Examples of protective measures of these kinds are given, for example, in EP 1 428 902 A1, EP 0 783 043 A1, US 2004/0170849 A1, U.S. Pat. Nos. 5,660,885, 7,780,832, 5,338,577, and 7,833,586, and in Julie M. Drexler et al., Air-Plasma-sprayed thermal barrier coatings that are resistant to high-temperature attack by glassy deposits, Acta Materialia 58 (2010) 6835 to 6844. The entire disclosures of the mentioned documents are incorporated by reference herein.
The last-mentioned publication proposes providing not only aluminum but also titanium in solid solution in yttrium-stabilized zirconium oxide, with corresponding ceramic coats being generated by atmospheric plasma spraying (APS) or suspension plasma spraying (SPS). The aim of incorporating aluminum and titanium into the glassy CMAS melt is to bring about crystallization, so that the glassy melt is no longer able to penetrate the thermal barrier coating. The aluminum and titanium, however, is in solid solution in the ZrO2 ceramic of the ceramic coat, and so first of all the aluminum and titanium must be taken up from the ceramic coat of the thermal barrier coating by the glassy CMAS melt. This, however, is disadvantageous for efficient provision of a protective effect relative to CMAS melts.
It would therefore be advantageous to be able to provide a thermal barrier layer or coating and a method for producing the same that affords effective protection against CMAS melts, it being at the same time simple to produce such a thermal barrier coating.